The most common way of processing signals is in a voltage mode where the information is contained in the amplitude of the voltage and the amount of current is irrelevant to the information. Voltage followers are well known in which the output voltage follows or equals the input voltage and in which the load of the output circuit is isolated from or separated from the input circuit in order to prevent the current drawn by the output load from affecting the magnitude of the input voltage.
It is also known to process signals in the current mode where the information is contained in the amplitude of the current and the amount of voltage needed to provide that current is irrelevant to the information. A current follower is an exact analogy to the voltage follower where the output current follows or equals the input current and in which the load of the output circuit must be isolated from or separated from the input circuit to prevent the load voltage necessary to provide the desired current from affecting the magnitude of the input current. An ideal current follower has an output current equal to the input current, a zero input impedance so that it has no effect on the input current, and an infinite output impedance.
High output impedance may be implemented by cascode shields, hob cancelling, etc., and accuracy in reproducing the current level may be obtained with Darlingtons, laser trimmed resistors, etc. The obtaining of a small input impedance has been more difficult. Generally, the prior art solution has been to use an operational amplifier, which is both large in volume and costly in its implementation, and which results in small but finite errors.
It is accordingly an object of the present invention to obviate the problems of known current followers and to provide a novel method and current follower with a theoretical zero input impedance.
It is another object of the present invention to provide a novel method and current follower which does not require an operational amplifier.
It is still another object of the present invention to provide a novel method and current follower which is readily implemented in an integrated circuit.
These and many other objects of the present invention are attained by use of a current mirror to provide one or more copies of the input current, and by the use of one of those copies to insure a zero input impedance.
Other objects, advantages and novel features of the present invention will become apparent from the claims and from the following detailed description of preferred embodiments of the invention when considered in conjunction with the accompanying drawings.